You restarted your modem, but did you power-off/power-on your Android device?
Can you ask your friend to come to your location, and try to connect their device through your WiFi? If they can connect, your WiFi is OK.
Has your Android device saved a password for the SSID of your WiFi? That password is saved for all your Android's apps (Weather? Web-browser? Twitter?). So, uninstalling/reinstalling the Spotify app probably won't make a difference with that password.
Can you tell your Android device to "forget" that connection's password, and then try again to connect to your WiFi, by selecting your network's SSID, and entering the password?